Re: [OAUTH-WG] JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens

Denis <denis.ietf@free.fr> Tue, 02 June 2020 08:20 UTC

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To: Benjamin Kaduk <kaduk@mit.edu>
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From: Denis <denis.ietf@free.fr>
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Subject: Re: [OAUTH-WG] JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens
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Hi Benjamin,

Responses are between the lines.

> On Fri, May 22, 2020 at 11:37:28AM +0200, Denis wrote:
>> Hi Benjamin,
>>> On Thu, May 14, 2020 at 04:29:43PM +0200, Denis wrote:
>>>> Since then, I questioned myself how a client would be able to 
>>>> request an access token that would be *strictly compliant with this 
>>>> Profile*.
>>> I don't understand why this is an interesting question to ask. The 
>>> access token and interpretation thereof is (AIUI) generally seen as 
>>> an internal matter between AS and RS, with the client having no need 
>>> to care about the specifics.
>> This document is*_a_* Profile for OAuth 2.0 Access Tokens. It is not 
>> _*the*_ Profile for *_all_ *OAuth 2.0 Access Tokens.
> Sure. But (in my understanding), in common usage, the contents and 
> interpretation of the access token is set by common agreement between 
> AS and RS, with the client serving only as a "dumb" channel for 
> transporting the token. That is, we're providing a token format that 
> an RS and AS can agree to use, most likely for all tokens issued by 
> the AS for that RS. I don't know of any existing mechanisms, or desire 
> for such mechanisms by deployments, for using a different token format 
> for different tokens issued by a given AS for a given RS.

Since this document is *_a_* Profile for OAuth 2.0 Access Tokens, it 
means that potentially other Profiles could be defined in the future.
In the request, there is no parameter for a client to indicate that it 
wants a JWT conformant to _this_ profile and no parameter either
in the response to indicate to the client that it got a JWT that is 
conformant to _this_ profile.

The processing mandated in the document of a request made by a client to 
an AS only applies for a request conformant to this profile
which may or may not include a scope parameter and which may or may not 
include a "resource" parameter (and, if it does not, shall
include an "aud" claim). Currently, it is not possible to make a 
difference between a JWT request or response conformant to this profile
and other JWT requests or responses.

> Attempting to have the client provide input that would affect such a 
> mechanism seems like complexity with no market demand; a "solution in 
> search of a problem" as it were. Is there some pent-up demand among 
> OAuth deployments that I'm not aware of? I freely admit to not being 
> very on top of the broad spectrum of what's deployed...
>> 1) A client may wish to obtain an Access Token that corresponds to 
>> this Profile because, for example, this document mandates the "sub" 
>> claim to be present". Hence, the content of the Access Token is not 
>> totally "/an internal matter between AS and RS/". However, I have not 
>> understood how a client could request an Access Token that 
>> corresponds to *_this_***Profile, since there is no mandatory 
>> parameter in the request (both the "scope" parameter and the 
>> "resource" parameter are optional). In the future, we could define 
>> _*another*_**Profile. Hence, there is the same question:  How could a 
>> client request an Access Token that corresponds to *_that 
>> other_***Profile ? 2) When getting a JWT,  how can the client make 
>> sure that the access token it got is compliant with this Profile ? 
>> RFC 8725 states in section 3.11 : 3.11. Use Explicit Typing 
>> Sometimes, one kind of JWT can be confused for another. If a 
>> particular kind of JWT is subject to such confusion, that JWT can 
>> include an explicit JWT type value, and the validation rules can 
>> specify checking the type (...). Explicit JWT typing is accomplished 
>> by using the "typ" Header Parameter. Wouldn't be wise to include an 
>> explicit JWT type value for JWTs conformant to this Profile ?
> In the model where the client is a "dumb" communications channel, this 
> question does not seem interesting. But the related question of "how 
> can the RS make sure that the access token it got was generated 
> according to this profile?" does seem interesting, and seems like it 
> would benefit from the same proposed solution.

An explicit JWT type value added both in the JWT request and in the JWT 
response would solve this problem.

>> This relates to an email posted by Dominick Baier under the topic 
>> "JAR: JWT typ" on May 19 : This has been brought up before - but no 
>> response. Either I can’t find it - or it is missing. But is the 
>> setting the JWT typ explicitly mentioned somewhere?
> It is fairly likely that I will remember to ask about explicit "typ" 
> usage if I'm still on the IESG when this document gets there: I think 
> I've been making a habit of doing so.

Once again, an explicit "typ" sould be considered for both the JWT 
request and the JWT response. This implies that the client "MUST" be 
able to inspect the content of the access token.

Denis

> Thanks, Ben